The present invention relates generally to chemical vapor infiltration and deposition (xe2x80x9cCVI/CVDxe2x80x9d) processes, and more particularly, to a hardware assembly for supporting porous structures during a CVI/CVD process.
Chemical vapor infiltration and deposition (xe2x80x9cCVI/CVDxe2x80x9d) is a well known process for depositing a binding matrix within a porous structure. The term xe2x80x9cchemical vapor depositionxe2x80x9d (xe2x80x9cCVDxe2x80x9d) generally implies deposition of a surface coating, but the term is also used to refer to infiltration and deposition of a matrix within a porous structure. As used herein, the term xe2x80x9cCVI/CVDxe2x80x9d is intended to refer to infiltration and deposition of a matrix within a porous structure. The technique is particularly suitable for fabricating high temperature structural composites by depositing a carbonaceous or ceramic matrix within a carbonaceous or ceramic porous structure resulting in very useful structures such as carbon/carbon aircraft brake disks, and ceramic combustor or turbine components.
Generally speaking, manufacturing carbon parts using a CVI/CVD process involves placing preformed porous structures in a furnace and introducing a high temperature reactant gas to the porous structures. A variety of porous structures and reactant gases may be used, but typically, a fibrous carbon porous structure is used with a reactant gas mixture of natural gas and/or propane gas when carbon/carbon aircraft brake disks are manufactured. As well understood by those in the art, when the hydrocarbon gas mixture flows around and through the porous structures, some of the carbon atoms separate from the hydrocarbon molecules, thereby depositing the carbon atoms within the interior and onto the surface of the porous structures. As a result, the porous structures become more dense over time as more and more of the carbon atoms are deposited onto the structures. This process is sometimes referred to as densification because the open spaces in the porous structures are eventually filled with a carbon matrix until generally solid carbon parts are formed. U.S. Pat. Nos. 5,480,578 and 5,853,485 to Rudolph et al., hereby incorporated by reference, also describe in detail additional aspects of CVI/CVD processes.
Densification processes for annular brake disks may be characterized as either conventional densification processes or rapid densification processes or variants thereof. In conventional densification, annular brake disks are arranged in stacks with adjacent brake disks stacked on top of each other. A center opening region is thus formed through the center of each stack. Typically, spacers are placed between adjacent brake disks to form open passages between the center opening region and the outer region. Thus, the reactant gas flows randomly around the stack and may flow through the open passages from the center opening region to the outer region or vice versa. As a result, the pressure differential between the inlet and outlet ducts of the furnace is usually relatively low in conventional processes. On the other hand, in rapid densification, the open passages between the center opening region and the outer region are sealed to constrict the flow of the reactant gas between the center opening region and the outer region. Therefore, the pressure differential between the inlet and outlet ducts of the furnace is higher than the pressure used in conventional densification. As a result, the high pressure differential forces the reactant gas to flow through the interior of the porous brake disk structures, thereby increasing the rate of densification compared to conventional processes. Conventional and rapid densification processes may also be combined to achieve optimum densification. For example, a rapid densification process may be used in a first densification to decrease densification time, and a conventional densification may be used in a second densification to improve densification quality.
One area of concern during densification is the distribution of the reactant gas flow through and around the porous structures. Gas flow distribution can have a significant impact on the quality of the densified carbon parts and also can affect the cost of production. For example, in one method disclosed in U.S. Pat. No. 5,904,957 to Christin et al., stacks of annular preforms are placed in a furnace with spacer elements placed between each of the preforms and between the last performs in the stacks and the screens at the top end. Thus, leakage passages are formed between adjacent preforms. The gas is then channeled towards only (i.e., exclusively) the interior passage of each annular stack at the bottom end. The top end of the stacks are closed by solid screens. One disadvantage with this method is that the outer surfaces of the brake disks near the bottom of the stacks may become starved for gas, thereby producing an undesirable densification of the bottom brake disks and nonuniformity between the bottom and top brake disks. Another disadvantage is that the closed top end of the stacks blocks the gas flow out of the top end, thus causing gas stagnation problems as described in detail below.
Another problem that often occurs during densification is soot and tar accumulations on surfaces of the brake disks and/or on the furnace equipment. As is known to those in the art, soot usually refers to undesirable accumulations of carbon particles on the furnace equipment, while tar usually refers to undesirable accumulations of large hydrocarbon molecules on the brake disks and the furnace equipment. Typically, accumulations of soot and tar form when the reactant gas stagnates for a period of time in an area or comes into contact with cooler furnace surfaces. Stagnation typically occurs in areas where the gas flow is blocked or where the gas flow is moving more slowly than the surrounding gas flow. Sudden temperature changes also contribute to the formation of oily substances, such as tar.
Accumulations of soot and tar can cause a number of problems which affect both the quality of the carbon parts and the costs of manufacturing. Seal-coating is one typical problem that can result from soot and tar accumulations, although seal-coating can also be caused by other conditions that are described below. Seal-coating can occur when soot and tar accumulates early in the densification process on surfaces of the porous structure. As the carbon and large hydrocarbon molecules accumulate on the surfaces of the porous structure, the surface pores eventually become blocked, or sealed, thus preventing the flow of reactant gas from further permeating the porous structure. As a result, densification of the interior region around the seal-coated surface prematurely stops, thereby leaving interior porous defects in the finished carbon part.
Maintenance costs also increase due to soot and tar accumulations on the furnace equipment. During the densification process, accumulations of soot and tar often form throughout the furnace equipment. As a result, an extensive manual cleaning process may be periodically required after each production run to remove all the accumulations and prepare the furnace for the next production run. This cleaning job can be very time consuming and can result in significant delays between production runs. The accumulations can also make disassembly of close fitting parts especially difficult since the accumulations tend to bind the parts tightly together. As a result, furnace equipment sometimes becomes damaged during disassembly due to the difficulty of separating the parts. Additionally, the furnace vacuum lines sometimes become constricted by soot and tar. As those in the art are familiar, the vacuum lines are used to generate the desired gas flow through the furnace. However, soot and tar accumulations sometimes build up in these lines and reduce the performance of the vacuum. Therefore, the vacuum lines must be regularly cleaned, which is a time consuming and expensive task.
In order to produce high quality, low cost parts, carbon deposition should be as uniform as possible around and through the porous structures. One way to achieve this desired uniformity is to optimize the residence time of the gas in the furnace. Residence time typically refers to the amount of time required for a gas to travel through the furnace or other designated area. Typically, a low residence time is associated with an unobstructed flow path and is generally preferred.
Gas flow obstructions often cause additional problems during densification. As previously explained, seal-coating is a common problem that causes porous defects within the interior region of the completed carbon parts. However, in addition to the causes previously described, seal-coating also can occur due to nonuniform carbon deposition. This typically occurs when a nonuniform gas flow accelerates carbon deposition at the surface of a part, thereby sealing the surface with carbon deposits and blocking gas diffusion into the interior of the carbon structure. Usually this type of seal-coating occurs later in the densification process when the density of the porous structures are higher.
Another problem associated with nonuniform carbon deposition is the formation of undesirable carbon microstructures. For example, in the case of high performance carbon/carbon brake disks, a rough laminar carbon microstructure is preferred because of the friction and thermal characteristics of this microstructure. However, when the residence time of the gas flow is prolonged or the gas flow stagnates in obstructed areas, smooth laminar and dark laminar carbon microstructures may form instead. As known by those in the art, smooth and dark laminar microstructures are generally undesirable because brake disk performance is reduced unless the outer surfaces containing the undesirable microstructures are machined off in subsequent operations.
Accordingly, a hardware assembly is provided for controlling the flow of gas through and around a stack of annular porous structures in a furnace during CVI/CVD processes. In one embodiment, a distributor is provided which separates the gas into a first portion and a second portion. The first portion of gas passes to a center opening region formed by the stack of annular porous structures. The second portion of gas passes to the outer region around the stack of annular porous structures. Most of the first portion of gas is blocked from completely passing out the top end of the center opening region by a plate and a cap. Some of the first portion of gas, however, is allowed to pass out the top end of the center opening region through holes in the cap and through an open passage between the top annular porous structure and the plate. In another embodiment, the first portion passes to the center opening region and the second portion passes to the outer region without the use of a separate distributor.
In another embodiment, an open space is provided between a floor plate and a bottom base plate. Most of the gas flows from the open space as a second portion to the outer region, while some of the gas flows from the open space as a first portion to the center opening region. Most of the second portion flows through open passages between adjacent porous structures thereby commingling with the first portion. The commingled first and second portions pass out of the center opening region through an exit hole adjacent one end of the stack of porous structures.
A method of densification is also provided with two densification processes. The flow of gas between the center opening region and the outer region is reversed in the two densification processes. In one embodiment, one of the densification processes is a rapid densification process.